1. Field of the Invention
This invention relates to an exchange system for connecting a plurality of extension terminals and executing processing for connecting calls to the extension terminals in accordance with incoming-call information from an outside line. The invention relates also to a method of connecting calls in such an exchange system.
2. Prior Art
In an exchange known in the art, a call is placed to a specific extension terminal based on specific terminal designating information when such designating information arrives. The terminal designating information accompanies an incoming subaddress of direct inward dialing (DID) or an ISDN, by way of example. There are cases where terminal information designated by the terminal designating information of the incoming-call information is not registered in such an exchange. When such is the case, the following processing is executed:
(1) A message informing of an erroneous input is sent to the terminal on the calling side.
(2) The line is cut off without giving a response.
(3) A call is placed to a certain specific extension terminal that has been provided for a case such as this (this is performed by a exchange operator or the like).
Even if information relating to an extension terminal designated by terminal designating information has been registered, there are cases where the terminal is busy and cannot receive a call. This is dealt with by adopting the following method:
(4) The terminal on the calling side is informed of the fact that the called terminal is busy.
(5) As in example (3) mentioned above, a call is placed to a specific extension terminal based on fixed information that has been registered.
An exchange of this kind accommodates a modem pool (a plurality of extension terminals sharing a modem). In an exchange designed so as to be connected to such a modem pool, the following processing is executed based on incoming terminal designating information:
(6) A connection is made based on the fixed information that has been registered.
(7) The modem pool is designated by a specific number code, and an operator selects it one by one.
However, when a call is placed to a specific terminal as in (3), (5) and (6) in the above-described examples of the prior art, the terminal attributes on the calling side are not known. An unfortunate consequence is that in a case where the calling side is, e.g., a facsimile machine and the called terminal is, e.g., a telephone where an exchange operator is present, the calling side is billed for the call without normal communication place. Further, in examples (1), (2) and (4), the call is wasted. In example (7), where an operator must select one by one, a problem encountered is that the operator may make a designation error.
In recent years an exchange has been developed which accommodates a network, typified by an ISDN, that is capable of giving notification of communication attributes as incoming-call information, and can register attribute information of each of extension terminals as managing information. However, even an exchange of this type does not deal sufficiently with the foregoing problems. Specifically, when a connection has been made to a network capable of services having a variety of communication attributes despite the fact that the network is a single network, as in the case of an ISDN, there are cases where the designated terminal is not connected because the terminal designating information (a subaddress or the like) is erroneous, or where the designated terminal cannot be connected, as when it is busy, even though the terminal designating information is valid. In such cases, performing call processing (which, in the case of a group telephone line, entails connecting all lines to the exchange operator) by relying solely upon the terminal designating information or trunk attributes is inadequate and the advantages of an ISDN cannot be fully exploited.
Furthermore, when an extension terminal has been set to an automatic transfer, all incoming calls are automatically transferred to this terminal and, by way of example, it is impossible to arrange it so as to transfer only an incoming call from a communication terminal having a certain specific terminal attribute. Consequently, in the prior art described above, non-coincidence of attributes between terminals easily occurs after a connection is made. In communication with a network such as one capable of matching communication attributes between terminals before connection, as in the case of an ISDN, it is necessary to register and designate communication terminals and modem pools so as to make the connection based on communication attributes. In addition, in the aforementioned arrangement wherein communication terminals and modem pools are fixed and registered, there is inadequate flexibility. Moreover, designation errors are frequent when designation is performed one by one.